Page 189 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
188 CHAPTER SIX

■ In areas likely to have expansive, compressible, or shifting soils
or other unknown soil characteristics, the building official may
require a soil test by an approved agency to determine soil char-
acteristics at a particular location.

■ When topsoils or subsoils are expansive, compressible, or shift-
ing, they must be removed to a depth and width sufficient to
assure stable moisture content in each bearing area or stabilized
within each bearing area by chemical treatment, dewatering, or
presaturation. Unstable soils that are removed may not be used
as fill in other areas.

■ Concrete must have a minimum compressive strength as shown
in Figure 6-1.

6.1.1 Soil-Bearing Pressures
The soil which supports building foundations must be strong enough
to withstand the loads that are applied to it. The Code provides that in
lieu of a complete soils evaluation to determine bearing characteristics,
the values in Figure 6-2 may be assumed. If you do not know what type
of soil exists on a given site, the building official should be able to tell
you what the code requirements are. You’ll need to know what the soil
bearing capacity is to determine minimum footing dimensions.

6.1.2 Frost Depth

The water in soil freezes and expands, then contracts again when it
thaws. This phenomenon is called frost heave. Footings and founda-
tions must be set below the winter frost line to avoid damage from frost
heave. The depth to which the soil freezes depends not only on cli-
mate and geographic location, but also on soil composition, altitude,
and weather patterns. The map in Figure 6-3 shows long lines of equal
frost depth in the central and southern states, but in the west and north
shows local frost depths that can vary widely within a small area.
Along the Gulf coast, the frost depth is only 1 in., but in northern
Maine a footing must be set 6 ft. deep to reach below the frost line.

6.2 Footings

Foundation walls can bear directly on the subsoil when the soil has a
high bearing capacity. If the soil bearing capacity is lower, the wall

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